The present invention relates to reciprocating compressors, and in particular to equipment for continuous regulation of the flow rate in the said compressors.
There are various possible methods of regulating the flow rate: devices external to the compressor which may be considered are on/off operation, variation of the speed of the motor driving the compressor, a by-pass between the delivery and inlet, and inlet throttling, while devices forming part of the compressor itself which may be considered are idle/load operation, backflow control and the introduction of an additional dead space which may be constant or variable.
Regulation by means of additional dead space is provided by adding a dead space to the cylinder to enable the opening of the pressure valves to be delayed, thus reducing the flow rate; it is possible to carry out either step regulation, by adding various dead spaces of different capacities, or continuous (stepless) regulation, by using an additional dead space of variable capacity, as indicated in U.S. Pat. No. 6,641,371.
Idle/load operation, which does not provide continuous regulation of the flow rate, is suitable when a storage reservoir is present in the system and a variation of the delivery pressure is acceptable; the pressure of the reservoir is controlled by a hysteresis regulator. Generally, the flow rate is regulated by actuators composed of pneumatic devices, which, by acting on a body (the pusher) present in each valve, enable the sealing element to be kept in a predetermined position (open), thus making the compressor idle (zero flow rate); when the said devices are inoperative, the compressor operates at maximum capacity.
The frequency of actuation of the pneumatic devices which operate the pushers of the inlet valves depends on the amplitude of the hysteresis, the volume of the reservoir and the maximum unbalance between the nominal flow rate and the minimum flow rate of the load; however, the said value must be limited to avoid excessive wear on the pneumatic devices.
This type of control of the flow rate of compressors causes a decrease of the global efficiency and of the power factor in the “idle operation” phase; furthermore, the heat generated in the “idle operation” phase is not dissipated, and thus increases the temperature of the sealing elements. Finally, the use of an actuator without position control, its limited response time and rise time, together with the presence of long pipes having limited cross sections and considerable dead space, and the absence of synchronization of the movement with the compressor shaft gives rise to a number of contacts at uncontrolled velocity between the sealing element and the pusher, which reduce the reliability of the valves, causing wear on the pusher and the breakage of the sealing element.
Backflow control is provided by delaying the closing of the inlet valve with respect to the closing point in the case of maximum flow rate. The gas which has entered the cylinder flows back into the inlet duct in a quantity proportional to the portion of the compression stroke during which the inlet valves are kept open.
The use of continuous regulation permits the use of storage reservoirs of limited capacity, since the pressure variations are practically absent. The actuation methods used up to the present time for controlling the position of the sealing element of the valves are of the pneumatic or oil hydraulic type.
Examples of some devices based on continuous backflow regulation are described in the documents U.S. Pat. Nos. 7,331,767 and 5,988,985. These devices use various actuation systems based on fluid which is supplied to a piston. Both systems require a panel for regulating the pressure of the fluid used for the actuation.